Process for removing reactive silica from a bayer process feedstock

ABSTRACT

A process for the removal of reactive silica from a feedstock to the digestion step of the Bayer process for the manufacture of alumina, in which (a) the feedstock is contacted with a caustic liquor under process conditions which result in dissolving and stabilizing at least 50% of the silica into solution at a level of at least 3 gpL and without significant precipitation from solution of dissolved silica, (b) the silica bearing liquor is separated from solid residue of step (a) under conditions which do not promote significant precipitation of the silica; and (c) aluminosilicates are precipitated from the separated silica bearing liquor from step (b), to form a solid aluminosilicate product which is free of a majority of all other components of the feedstock.

BACKGROUND OF THE INVENTION

Bauxite is the major source of aluminium containing ore used in theproduction of alumina. Bauxite contains hydrated forms of aluminiumoxide (alumina) that occur in several different structural forms,depending upon the number of molecules of water of hydration and thecrystalline form. Most commercially useful deposits of bauxite includegibbsite (alumina trihydrate) and/or boehmite (alumina monohydrate)and/or diaspore.

Alumina is extracted from bauxite by use of the Bayer process. Briefly,the Bayer process includes the steps of contacting bauxite with a hotcaustic solution to dissolve alumina therefrom. If the bauxite containsmainly gibbsite, extraction of alumina from the bauxite may be conductedusing a caustic solution at a temperature generally in the range of 100to 150° C. If the bauxite contains mainly boehmite, or diaspore highertemperatures, in the order of 200 to 300° C. are generally required. Formixed bauxites containing both gibbsite and boehmite, a double digestionprocess may be used.

After digestion, the bauxite/caustic solution mixture is separated intoa pregnant liquor containing dissolved alumina (usually in the form ofsodium aluminate) and a solids residue (usually referred to as red mud).The pregnant liquor is fed to a precipitation circuit where it is cooledand seeded with solid particles of alumina trihydrate to induceprecipitation of alumina trihydrate from the pregnant liquor. Theresulting precipitation slurry is separated into a spent liquor streamand a solids stream. Coarse solids represent product and are transferredto a calcination stage where they are calcined to produce alumina. Finesolids are returned as seed particles to the precipitation circuit. Thespent liquor is returned to the digestion step where it is contactedwith further bauxite. Between the digestion and precipitation steps,there is generally one or more washing steps and the spent liquorgenerally must be evaporated to obtain the required causticconcentration prior to being returned to the digestion step. The Bayerprocess has been used commercially for about 100 years and it is wellknown to persons of skill in the art.

Bauxite, in addition to containing hydrated forms of alumina, includesseveral impurities. The main impurities are compounds of iron, titaniumand silica. The compounds of iron and titanium found in bauxitegenerally are insoluble in caustic solutions and have little effect onthe selective extraction of alumina from bauxite. These compounds reportto the red mud following digestion.

The silicon compounds present in bauxite occur mainly as quartz and ashydrated double salts with alumina, such as kaolin. Quartz dissolvesslowly in caustic solutions and the other forms of silica in the bauxitemay dissolve rapidly in the caustic solutions used in the digestionstep. Accordingly, bauxites containing significant amounts of silicahave the potential to be difficult to treat.

The presence of silica in bauxite can cause at least two major problemsin the digestion of bauxite, these being:

(i) dissolution and reprecipitation of silica as complex sodiumalumino-silicates, thereby consuming caustic soda; and

(ii) reprecipitation of complex sodium alumino-silicates on plantsurfaces, thereby causing scale build-up. This problem is especiallysevere when scale builds up on heat exchange surfaces.

Prior art attempts to deal with problems associated with silica inbauxite have concentrated on either suppressing silica dissolution orcompleting precipitation of silica in a controlled step to minimisescaling throughout the remainder of the plant. The problems of causticsoda consumption and scaling of plant surfaces are largely independentof each other—a low silica bauxite will result in low caustic sodalosses but can drive significant scaling problems while a high silicabauxite will consume large quantities of caustic soda but will result inless scaling. It is for this reason that most of the prior art dealingwith the impact of silica on the Bayer process deals with only oneaspect of the problem. The prior art can be grouped broadly into fourareas, as discussed below:

1) Predaesilication

A significant number of refineries include a so-called predesilicationoperation prior to digestion where the bauxite is held at a temperatureof around 100° C. for 6-18 hours. The purpose of this operation is toconvert a large portion of the reactive silica to sodalite type sodiumalumino-silicate which will then act as seed to rapidly convert theremaining reactive silica to sodalite type sodium alumino-silicateduring digestion. The conditions under which predesilication isconducted, low caustic and alumina concentration, ensure that only avery small proportion of the total reactive silica is in solution at anygiven time. The primary purpose of predesilication is to ensure thatconversion of reactive silica to sodalite type sodium alumino-silicateis complete so that pregnant liquor from digestion contains a minimumamount of dissolved silica which in turn minimises alumina productcontamination and sodium alumino-silicate scaling during subsequentreheating of spent liquor. This operation has no impact on the amount ofcaustic soda consumed as a result of silica reaction. Sodiumalumino-silicates are usually discarded from the Bayer plant as acomponent of red mud. However, a separate sodium alumino-silicateprecipitation step after predesilication has been proposed.

U.S. Pat. No. 3,413,087 in the name of Roberts, assigned to ReynoldsMetals Company, describes a Bayer process for extracting alumina frombauxite which includes a predesilication step to dissolve silica priorto digestion. In the predesilication step, the bauxite is mixed withspent liquor or strong liquor containing make-up caustic. The quantityof caustic present in the liquor is insufficient to dissolve all of thesoluble alumina in the bauxite but is sufficient to dissolvesubstantially all of the soluble silica in the bauxite. However, only asmall fraction of the soluble silica is in solution at any time. Theslurry (of bauxite and liquor) is maintained in the predesilicationstage (called a predigestion stage in the patent) at a temperature offrom 150° F. to the temperature used in the digestion step for a periodof time (e.g. 30 minutes to 12 hours) to allow the dissolved silica tocrystallize and precipitate as a complex sodium aluminium silicatedesilication product. The patent states that crystallization ofdesilication product (DSP) causes the dissolved silica to preferentiallyprecipitate on the DSP particles, rather than on other surfaces such asheat exchange surfaces. The turbulence of the slurry in the digestionsystem can also act to maintain clean heat exchange surfaces. The DSP isinsoluble and allows the slurry to pass to the digestion stage withoutscaling of heat exchange surfaces occurring. After digestion, the DSP isremoved in the red mud residues.

A paper by Eremin, from the USSR Institute of Mining, Leningrad,entitled “Beneficiation of Low Grade Bauxite by HydrometallurgicalMethods” (in: Proc. Conference; Alumina Production until 2000, Tihany,Hungary, Oct. 6-9, 1981, p. 135-142.) discloses studies into thedissolution of silica components from bauxite. Following these studies,the paper concluded that bauxite desilication should be carried out atapproximately 80 to 90° C. at a high liquid to solids ratio and withmedium caustic concentrations (100 to 150 g/l Na₂O, which corresponds to170 to 260 g/l, calculated as Na₂ CO₃). This paper makes reference to atreatment step in which a portion of the reactive silica is dissolvedbefore Bayer process digestion. The reactive silica which enterssolution is subsequently precipitated to produce a separablealuminosilicate material. However, this paper highlighted a majorlimitation in carrying out the process, since stable silica levels insolution never exceeded those expected in recycled Bayer liquors by morethan about 2.5 gpL, limiting the effectiveness of silica dissolution atrealistic liquor to bauxite ratios for bauxites having high silicacontents. Thus in the best tests reported by Bremin only about 50% ofthe reactive silica was removed. For this reason Eremin proposed the useof a bauxite precalcination step to activate the silica and deactivatethe alumina in the bauxite to a degree so that better selectivity couldbe obtained.

2) Low Temperature Digestion

A number of processes have been disclosed which aim to reducedissolution of reactive silica. These processes are based on a lowtemperature digestion of bauxite where the very short residence time isjust sufficient to extract alumina but insufficient to completelyconvert reactive silica to DSP. The temperatures required for aluminaextraction is such that the desilication reaction still proceeds veryrapidly so that it is not practically possible to achieve a solid-liquidseparation for the purposes of substantial silica removal from the solidresidue when the silica content of the liquor is at its highest. Inorder to minimise silica conversion to DSP or dissolution it isnecessary to limit the digestion temperature to a maximum of 150° C.meaning that this process is only suitable for gibbsitic bauxites. Onlylimited reduction in caustic consumption is achieved through reducedsilica dissolution. Pregnant liquor must be separately desilicated aftermud separation as it contains a high level of silica (typically 2-3g/l).

U.S. Pat. No. 4,661,328 in the name of Grubbs, assigned to AluminiumCompany of America, describes a process for purifying an alumina richore containing more than 5 weight percent reactive silica. The processincludes the steps of mixing the ore with an aqueous digestion solutionof silica and sodium aluminate and digesting the mixture at atemperature of 80 to 150° C. to dissolve alumina whilst inhibiting thedissolution of reactive silica from the ore. The bauxite may be mixedwith an aqueous digestion solution that is nearly saturated with silicaand nearly saturated with alumina. In this process, the high silica oreis mixed with an aqueous digestion solution having high alumina, highsilica and high soda in solution. The silica is present in the digestionsolution at a concentration of greater than 1.8 g/l, typically 1.8 to2.5 g/l. A post desilication process, seeded by desilication product, isrequired. Alumina may be present in the solution in an amount of from150 to 170 g/l while soda values in the solution typically fall withinthe range of 240 to 300 g/l, calculated as Na₂ CO₃. The temperatures inthe digestion step are lower than in conventional Bayer processtemperatures and they typically fall within the range of 80 to 150° C.,most preferably 100 to 120° C. This process retards or avoids thedissolution of silica from bauxite. The process can only be used withgibbsitic bauxite as the digestion temperatures utilised in this processare too low to viably digest boehmite containing bauxites.

U.S. Pat. No. 3,716,617 in the name of Oku et al, assigned to SumitomoChemical Co Limited, relates to a bauxite digestion process in which adigestion residue comprising solid components in a slurry that has notbeen subjected to a desilication treatment is separated by use of asynthetic organic high molecular weight flocculent from a sodiumaluminate liquor resulting from bauxite digestion. The separation isconducted whilst at least 5% of the reactive silica remains unconvertedto DSP in the solid residue. In the digestion step of this patent, thesoda content may fall within the range of 80 to 200 g/l. The temperatureis preferably 90 to 150° C. The process is based upon operating thedigestion step to reduce the amount of reactive silica dissolved fromthe bauxite. The combination of temperature and residence time in theprocess is chosen to minimise silica dissolution. Typical operatingparameters for the digestion step include operating at a temperature of110° C. for ten minutes, or operating at 140° C. for 60 seconds. Theprocess described in Oku et al avoids complete silica dissolution orconversion to DSP during bauxite digestion.

Japanese Patent Application No. Sho 62-230613 discloses a process forextracting alumina from reactive bauxite in which the temperature andresidence time are controlled to obtain substantially complete aluminadissolution, but only a small amount of the silica in the bauxite isextracted into solution or converted to DSP. After the requireddigestion time, the digestion slurry is rapidly cooled to quench thesilica dissolution reaction. This process relies upon the differences inthe rates of dissolution of alumina and silica to minimise the amount ofsilica extracted into solution or converted to DSP. Any silica that doesgo into solution is removed by seeding with sodalite to promote sodiumalumino-silicate precipitation.

WO 93/20251 in the name of Sumitomo Chemical Company, Limited disclosesa bauxite digestion process in which bauxite and an alkaline solutionare mixed to form a slurry. The slurry is fed to an extractor andalumina is extracted whilst dissolution of reactive silica issuppressed. The solid residue is separated from the liquor without anyof the reactive silica dissolved in the liquor being precipitated as asodium alumino-silicate. The operating conditions in the extractorinclude an Na₂O content in solution of 100 to 160 g/l, a temperature of110 to 160° C. and a residence time of up to 10 minutes. As alumina isextracted from the bauxite, the liquor fed to the extractor must have arelatively low dissolved alumina content. In the extractor, theextraction of reactive silica does not exceed more than 70%, preferablynot more than 5 by weight. The re d mud recovered from the digestionstep contains a substantially reduced amount of sodium alumino-silicate.The liquor recovered from the digestion step contains only a limitedamount of the original silica in the bauxite, and it is desilicated byadding a solid silicate material as seed to form insoluble silicatematerials, such as sodalite or zeolite. Prevention of sodiumalumino-silicate precipitation in the extractor is achieved by using ashort residence time in the extractor.

U.S. Pat. No. 5,122,349 discloses a gibbsitic bauxite digestion processin which the gibbsite is rapidly dissolved to reduce the free hydroxideconcentration. This leads to a reduced kaolinite dissolution. Thisprocess is only applicable to gibbsitic bauxites.

3) Double Digestion

Double digestion is a process designed for mixed, gibbsitic/boehmiticbauxites which allows boehmite to be extracted at a lower temperaturethan required in a normal high temperature plant. The effect of this isthat dissolution of a portion of the silica in the form of quartz can bereduced. However, quartz usually only accounts for a small proportion ofthe total reactive silica in bauxite. Conversion of silica in kaoliniteto DSP cannot be avoided in a double digestion process if the bauxite ishandled as a single stream. However, in the unusual case that bauxitecan be beneficiated to give gibbsite/kaolinite and boehmite fractions,caustic consumption can be reduced in the low temperature digestion. Theseparation of some silica (or sodium alumino-silicate) is possible afterthe low temperature digestion stage by keeping the temperature andresidence time to a minimum and then desilicating the pregnant liquorafter mud separation.

U.S. Pat. No. 4,994,244 in the name of Fulford et al, assigned to AlcanInternational Limited, relates mainly to a digestion process thatseparates red mud from liquor in a pressurised vessel. However, a sidebenefit from the process described in Fulford is that it is possible tolimit the residence time of the bauxite in the digestion step to thattime actually required for extraction of alumina from the bauxite.According to Fulford et al, conventional digestion processes carry out asignificant part of the liquor desilication operation inside thepressure digester. As the liquor desilication reaction is relativelyslow, the liquor desilication reaction typically controls the residencetime required in the digester. With the process of Fulford et al, littleor no liquor desilication occurs in the digester and a seeded,controlled desilication of the liquor can be carried out after thepressurised red mud separation step. This is, of course, a postdesilication reaction. It is noted that one variant of the Fulford et alprocess (FIG. 6 in U.S. Pat. No. 4,994,244) utilises a double digestionstep with desilication occurring in the digester used for gibbsiticdigestion. The patent does not specify the properties of the causticliquor used in the digestion, except to say that typical liquors forBayer process digestion are used. The short residence time for digestiondisclosed in Fulford et al allows the removal of the already smallamount of quartz in the bauxite to be minimised and it also allows forsolid/liquid separation before significant quantities of sodiumalumino-silicate solids have formed in the digestion step.

U.S. Pat. No. 4,614,641 in the name of Grubbs, assigned to AluminumCompany of America, describes a digestion process for gibbsiticbauxites. The bauxite is ground and separated into a coarse fraction anda fine fraction. The fine fraction is subjected to a low temperaturedigestion at 80 to 120° C. to produce a digestion solution containingsome silica (greater than 1.8 g/l) but a significant proportion of thesilica in the fine bauxite remains unconverted and unextracted. Theproduct digestion solution also has a high alumina content (nearsaturation) and a caustic concentration of more than 240 g/l (asNa₂CO₃).

The coarse bauxite fraction is digested under higher temperatures andpressures. The mixture of solids and liquor resulting from digestion ofthe coarse fraction contains desilication product, which evidentlyprecipitates in the high pressure digester. This mixture is thencontacted with the clarified liquor containing some silica (up to 1.8g/l) from the low temperature digestion (in either the high pressuredigester or the clarifier line), and the overall mixture is subsequentlyclarified. The fine fraction liquor is desilicated by contacting thefine fraction liquor with the mixture of solid residue and liquor fromthe coarse fraction digestion. The overall process is based upon thedifferences between the rates of dissolution of gibbsite and kaoliniteas a function of temperature and bauxite residence contact time. It isalso noted that the caustic solution used to contact the fine fractiondigests substantially all of the gibbsitic alumina, from the finefraction of the bauxite, and most of the silica remains undigested.Accordingly, this solution must initially have a low alumina content(before contacting the fine fraction of bauxite).

4) Sinter Process

This is an alternative to the Bayer process, commonly used in Chineseand Russian refineries for treatment of very high silica bauxites ornon-bauxitic ores such as nepheline. A general description of sintertechnology is given in a paper by B. I. Arlyuk and A. I. Pivnev in LightMetals 1992, pages 181 to 195. The ore is calcined with soda and limeunder conditions where the products are sodium aluminate and calciumsilicate. This mixture is then rapidly leached in water or spent liquorto recover the soda and alumina. During leaching some of the calciumsilicate also dissolves leading to very high silica contents in theleach liquor (up to 15 g/l). This liquor is subsequently desilicatedprior to precipitation of alumina. The leaching conditions are set tominimise silica dissolution and conversion to desilication product soonly a small fraction is taken up.

In summary, in prior art processes for dealing with the problems of highsilica feeds to the Bayer process either:

the bulk of the silica is chemically combined with high usage ofreagents such as lime or soda, and deported as solid alkali and alkalineearth silicates into a general mud residue (containing many othercontaminants and having poor handling characteristics) from whichchemical values cannot easily be recovered, or

by use of very short contact times a limited fraction of the non quartzsilica is deported unconverted to residue, thereby reducing by thisfraction caustic consumption in the formation of sodium aluminosilicatereporting to mud residue, but only where this silica is contained in agibbsitic bauxite or bauxite fraction, or

a limited fraction of the silica is taken into solution, and afterseparation of liquor from mud residue, is precipitated from liquors as aclean aluminosilicate, facilitating the recovery of chemical values fromaluminosilicate via subsequent techniques.

All of the prior art processes suffer from one or more disadvantages. Inparticular, there is no prior art process which results in thesubstantial avoidance of economically significant reagent consumptiondue to silica in Bayer feeds, or which facilitates the substantialdeportment of silica in Bayer feeds into separable clean streams fromwhich chemical values can be easily recovered.

SUMMARY OF THE INVENTION

After carefully reviewing the prior art, the present inventors haveestablished the need to develop an economically effective process whichis capable of transferring a large proportion of the reactive silica inbauxite directly into solution for a period sufficient to enable a solidliquid separation to be carried out. The silica could then be recoveredas a clean product for sale or further processing. It is an object ofthe present invention to provide such a process.

The present inventors have now proposed a process which meets this need,for which prior art processes have proved to be deficient.

In general, the present invention provides a process for removing silicafrom bauxite including the steps of mixing bauxite with a caustic liquorto form a mixture and to dissolve at least a substantial part of thereactive silica from the bauxite, said caustic liquor having a highcaustic concentration, and separating the mixture into a solidscontaining component and a liquor having a high alumina content andcontaining a high level of dissolved silica.

In more specific terms, the present invention provides a process for theremoval of reactive silica from feedstocks to the digestion step of theBayer process for the manufacture of alumina comprising the followingsteps:

(a) contacting the Bayer process feedstock with a caustic liquor underthe following process conditions which result in dissolving andstabilising at least 50% of the silica into solution at a level of atleast 3 gpL and without significant precipitation from solution of thedissolved silica:

(i) process exit liquor composition:

caustic strength (as Na₂CO₃) greater than 250 gpL

alumina concentration (as Al₂O₃) greater than 125 gpL.

(ii) temperature: 60° C. -125° C.;

(iii) contact time: greater than 20 minutes; and

(b) separating the silica bearing liquor from the solid residue ofsilica dissolution step (a) under conditions which do not promotesignificant precipitation of the silica.

The key requirements of the process of the present invention are use ofa liquor having a high caustic concentration to allow rapidsolubilisation of reactive silica and a high alumina concentration tostabilise a high level of silica in solution for a period of timesufficient to effect a solid/liquid separation. Advantageously, a lowtemperature is used to avoid accelerating precipitation of sodiumalumino-silicate prior to solid/liquid separation.

The present inventors have surprisingly found that contacting bauxitecontaining reactive silica with a caustic liquor containing a highcaustic concentration and a high alumina content can result in asubstantial part of the reactive silica in the bauxite going intosolution to a high concentration (typically between 7 and 15 g/l).Moreover, unlike prior art processes, in which sodium alumino-silicatebegins to precipitate after a relatively short time, the process of thepresent invention can result in the dissolved silica remaining insolution for appreciable periods of time, for example, in excess of 2hours, whilst also avoiding precipitation of sodium alumino-silicate onthe solids component. This allows adequate time to both dissolve thesilica and separate the silica containing liquor from the bauxite, aseparation that could not be achieved at these levels of silica insolution by prior art processes.

In a preferred embodiment, the present invention further includestreating the liquor containing dissolved silica after separation fromthe solids to recover a solid desilication product therefrom. As theliquor containing dissolved silica is separated from the solidscomponent prior to treating the liquor to recover the desilicationproduct therefrom, the desilication product may be recovered as a cleansiliceous product that may be further processed for caustic recovery orother purposes, or sold. The solid desilication product may be anyaluminosilicate known to be capable of precipitating from causticliquors by adjustment of temperature, liquor strength and composition,additives or seed. The ability to do further processing of a cleansiliceous product without the complications associated with othertypical red mud components gives several advantages, including theability to closely control the chemistry in subsequent processingbecause the feed would be consistent in composition, and the ability toproduce a range of clean siliceous products.

In an especially preferred embodiment, the process comprises a silicadissolution process which removes a substantial part of the reactivesilica from the bauxite prior to subjecting the bauxite to Bayer processdigestion. In this embodiment, the solids containing component from thesilica dissolution step is subsequently fed to a Bayer process digestionstep to extract alumina therefrom. Accordingly, it is preferred thatcontact of the bauxite with caustic solution in the silica dissolutionprocess of the present invention is conducted such that dissolution ofalumina from the bauxite into solution is minimised. It is important tonote that the failure to use high alumina liquors in the process canresult in premature and uncontrolled precipitation of sodiumalumino-silicate. For this reason, solutions with significant capacityto uptake alumina at the temperatures of the process should be avoided.

Minimisation of alumina dissolution in the silica dissolution processresults from using a caustic liquor having a high dissolved aluminacontent. The preferred caustic liquor for use in the silica dissolutionprocess comprises either a pregnant liquor or a caustic liquor at closeto saturation with alumina. Such liquors have limited capacity todissolve any further alumina at the temperatures proposed for the novelprocess, thus ensuring that minimal alumina will dissolve from thebauxite.

The liquor containing dissolved silica from the silica dissolutionprocess contains more than 3 g/l silica in solution, preferably 6 g/l to15 g/l dissolved silica in solution.

The step of contacting the bauxite with the caustic liquor results inmore than 50%, preferably more than 80%, and more preferably more than90% of the reactive silica in the bauxite going into solution.

The contact time of the bauxite and the liquor is at least 20 minutes,preferably at least 1 hour, more preferably 2 to 4 hours. These figuresare significantly higher than the silica dissolution and stabilitylevels reported in any prior art.

The process can be tailored to suit subsequent Bayer digestionprocesses. For example, sufficient silica can be left with the bauxiteresidue to seed liquor desilication through digestion, so minimisingscale build up.

The caustic liquor used in the silica dissolution process of the presentinvention has a high caustic concentration of greater than 250 g/l,preferably greater than 300 g/l, more preferably greater than 350 g/l,calculated as Na₂CO₃ (throughout this specification, all causticconcentrations are calculated as Na₂CO₃ unless otherwise specified).

The caustic liquor in the process also has a high dissolved aluminacontent of greater than 125 g/L, preferably greater than 150 g/l, morepreferably greater than 175 g/l (calculated as Al₂O₃).

The operating temperature of the process of the present invention iswithin the range of 60° C. to 125° C., preferably within the range of80° C. to 105° C. In the preferred temperature range, the temperature isbelow the atmospheric boiling point of the liquor and thus the need touse a pressure vessel for silica dissolution may be avoided. Iftemperatures substantially above the upper limit of the preferred rangeare used, the risk of spontaneous formation of sodium alumino-silicateDSP increases. For this reason, the temperature preferably is maintainedbelow 125° C.

The liquor fed to the silica dissolution step preferably has a lowdissolved silica content, for example, less than 2 g/L dissolved silica.More preferably, the dissolved silica content of the feed liquor is lessthan 1 g/L.

The solids loading of the bauxite/caustic liquor mixture preferablyfalls within the range of 50 g/L to 700 g/L, which corresponds to aweight percentage of solids in the mixture of 5 to 60%.

The present inventors believe that the process of the present inventionwill be effective in removing silica from bauxite using conditionsoutside the preferred ranges given above. In particular, lower causticconcentrations may be used. However, the extraction of silica from thebauxite may be lowered if operating conditions outside the preferredranges are used.

The present invention provides a process for removing silica frombauxite that may allow for substantially complete removal (certainly inexcess of 80% removal) of reactive silica from the bauxite. Moreover,the dissolved silica stays in solution for sufficient time to enablesolid/liquid separation to take place without substantial precipitationof sodium alumino-silicate on the bauxite.

As indicated above, the present inventors believe that the key to thisprocess is using a caustic liquor having a high caustic concentrationand a high content of dissolved alumina. This is a surprising resultbecause the sodium alumino-silicate that forms in a desilication step isa sodium alumino-silicate solid product.

Prior to the present disclosure there was no reason for a person skilledin the art to suspect that the stable supersaturation level of silicawill increase with increasing solution alumina content. The aluminacontent of the liquor stabilises silica into solution for useful timesin the context of solid/liquid separation at levels which can exceed thesolubility level by several times the solubility limit.

In view of the results obtained from experimental work on the presentinvention, the inventors have postulated that the presence of dissolvedalumina in the caustic liquor assists in stabilising the reactive silicainto solution by formation of complexes with aluminium species insolution.

The deficiencies of prior art compared to our process are summarisedbelow:

1) For conventional predesilication processes, in which silica isdeliberately fully converted to aluminosilicate desilication product themaximum dissolved silica levels are low (normally well less than 50% ofthe available silica is in solution at any point) and solution stabilitytimes are short. Therefore, the extraction of silica from bauxite is low(and subsequent solid/liquid separation is normally not practised).

2) For low temperature digestion processes, aimed at avoiding somesilica conversion to sodium bearing compounds and applicable only togibbsitic bauxites, high dissolved silica levels are not maintained forany useful time.

3) For double digestion processes, the problems associated withconventional predesilication and low temperature digestion are notavoided, and there is no other benefit which would assist in thetreatment of high silica bauxites.

4) For sinter processes, maximum dissolved silica levels are high, butstill represent only a small fraction of total silica in bauxite. Also,the bauxite requires pretreatment (roasting with additives), and isgenerally thought to be uneconomic for future alumina developments.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the accompanying drawings in which:

FIG. 1 is a schematic flow diagram showing one process embodying theinvention;

FIG. 2 is a schematic flow diagram showing another process embodying theinvention.

FIG. 3 is a graph showing silica concentration versus time for severaldifferent digestion processes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the process of FIG. 1, bauxite 10 is supplied to a silica dissolutionvessel 12 where the bauxite 10 is mixed with pregnant liquor 14 from thebauxite digestion step. In silica dissolution vessel 12, the pregnantliquor 14 acts to dissolve a substantial portion, for example,approximately 90%, of the reactive silica from the bauxite. After asuitable residence time, closely controlled to minimise the formation ofsodium alumino-silicates, the mixture from vessel 12 passes to solidliquid separator 16. The solids containing component 17 passes to adigester 18 where the desilicated bauxite is contacted with spent liquor20 to digest alumina therefrom. It will be appreciated that digestionstep 18 may comprise a single digestion or a double digestion, dependingupon the bauxite type being used. After bauxite digestion, the slurry istransferred via line 19 to a solid/liquid separator 21 where pregnantliquor 14 is separated from red mud 41.

The liquor 22 from solid/liquid separation 16 contains dissolved silicaand is preferably treated to remove silica therefrom. This may beachieved by treating the liquor 22 in silica precipitation vessel 23. Inthis vessel, liquor 22 is reacted , e.g. mixed with water and possiblyreagents 24 (and preferably seed material), possibly with heating, and adesilication product is formed. The result in precipitation slurry 25 issent to solid liquid separator 26 and a desilication product 27 isrecovered. Liquor 22 may also include a side stream 28 that is sent to afinal silica precipitation vessel 29. Liquid 30 from solid/liquidseparator 26 may also be fed to final silica precipitation vessel 29. Inthis vessel, a sodalite sodium alumino-silicate seed is added to resultin the precipitation of a sodalite sodium alumino-silicate. Theresulting precipitation slurry 31 is fed to a solid liquid separator 32and a precipitated product 33 is recovered. The liquor 34 is desilicatedpregnant liquor and it is passed to alumina precipitation circuit 35that may comprise any suitable process for precipitating alumina by theBayer process. The precipitation slurry 36 is classified in classifier37 in accordance with known Bayer process techniques. The aluminatrihydrate 38 recovered from the classifier includes product trihydrateas well as seed trihydrate for return to the precipitation circuit. Thespent liquor 39 is passed to evaporator 40 for concentration to asuitable level for use in bauxite digestion and it is then passed asspent liquor stream 20 to bauxite digester 18.

It will be appreciated that the flow sheet shown in FIG. 1 may besusceptible to one or more modifications. In particular, the process maydo without dilute silica precipitation vessel 23 or silica precipitationvessel 29. Furthermore, pregnant liquor stream 14 may be only partiallyfed to silica dissolution vessel 12. For example, pregnant liquor stream14 may be split with only part of the pregnant liquor being fed tosilica dissolution vessel 12 and part being fed to alumina precipitationcircuit 35.

FIG. 2 shows an alternative circuit to that shown in FIG. 1. In FIG. 2,bauxite 40 and recycled caustic liquor 42 having a high causticconcentration and a high dissolved alumina content are mixed in silicadissolution vessel 43. Any make up additives that may be required areadded through line 44. The mixture from vessel 43 is sent to phaseseparator 45 where the solids component 46 is separated therefrom andsent to the Bayer digestion process. The liquor 47 containing dissolvedsilica is sent to silica precipitation vessel 48 where a soliddesilication product may be precipitated, for example, by heating theliquor or by diluting the liquor, preferably with seed and possiblyother additives. The precipitation slurry 49 is sent to settler 50 wherea clean desilication product 51 is recovered therefrom for sale, orfurther treatment to recover caustic therefrom. The liquor 52 flows intorecycle stream 42. A bleed to a Bayer precipitation process 53 may alsobe incorporated into this stream.

It will be appreciated that the flowsheet of FIG. 2 will ensure that theliquor which circulates between silica dissolution and precipitation hasa high alumina content, as it will stabilise to a steady statecomposition which reflects the solubility of the bauxite alumina bearingphases.

Finally, throughout this specification, reference is made to alumina insolution. Dissolution of alumina in caustic liquors results in theformation of sodium aluminate and it will be understood that referencesto alumina in solution or dissolved alumina include sodium aluminatewithin this scope.

EXAMPLES

The bauxite used in these examples was a high silica blended Weipabauxite, milled to passing 0.5 mm, having the following composition (inweight percentage of the components):

Al₂O₃ Fe₂O₃ SiO₂ TiO₂ ZrO₂ SO₃ Na₂O LOI Sum quartz 55.5 11.8 5.8 2.6 0.10.2 0 24.4 100.6 1.4

FIG. 3 is a graph showing silica concentration against time for severaldifferent digestion processes, including the process according to theinvention (run 2 and run 3).

Table 1 summarises the experimental conditions for the testwork whoseresults are presented in FIG. 3.

The experiments were carried out in batch mode. The liquor compositionis expressed according to both alumina industry convention and chemicalcomposition. The values are for the starting liquor. The compositionchanged slightly during the runs, in particular the aluminaconcentrations increased by up to about 40 gpL.

Run 1 was conducted under conditions relevant to a conventionalpredesilication process in which the silica concentration increased to avalue of the order of 3 gpL, corresponding to less than about one thirdof the reactive silica in the bauxite, then decreased as desilicationproduct (sodium alumino-silicate) precipitates. This result does notrepresent adequate extraction of silica, especially given that in manysuch processes the starting liquor will not be free of silica. Inaddition, the solution was not sufficiently stable to ensure that solidliquid separation can be effective in ensuring good extraction ofsilica.

TABLE 1 Test Conditions For The Results of FIG. 3. Sumitomo Run 1 Run 2Run 3 digest Temperature  95° C.  95° C.  95° C. 130° C. Bauxite charge200 g/l 200 g/l 200 g/l 190 g/l Liquor Composition Caustic soda (CS) 250400 400 260 (g/l Na₂CO₃) alumina to caustic  0.4  0.6  0.4  0.32 ratio(A/C) caustic to soda  0.85  0.98  0.98  0.85 ratio (C/S) [NaOH] g/l 189302 302 196 [Al₂O₃] g/l 100 240 160  83.2 [Na₂CO₃] g/l  44  8  8  46(carbonate)

The line in FIG. 3 labelled “Sumitomo Digest” shows the silica profileobtained for a simulated digestion process according to some details ofSumitomo Patent Application WO93/20251. In this digestion process, thesilica concentration reached a very rapid peak and equally rapidlydropped away to a low level of silica concentration. In this process itis extremely difficult, if not impossible, to control the process to theextent necessary to allow the separation of a high silica liquor fromresidual solids free of desilication product.

A comparison of the three curves helps to distinguish the presentinvention from prior art. With conventional predesilication (run 1), themaximum silica concentration in liquor only represents a small fraction(˜30%) of the reactive silica in bauxite. With the Sumitomo typedigestion, a large fraction of the silica in bauxite is dissolved, butthe silica concentration is not maintained for any useful time.

In the graphs labelled run 2 and run 3, the process according to theinvention is illustrated. In run 2, silica concentration increasedcontinuously over about 4 hours, reaching a maximum of the order of 8gpL, which represents approximately 90% of the reactive silica contentof the bauxite sample. In run 3, the silica concentration reached amaximum of the order of 8 gpL and then decreased because of sodiumalumino-silicate precipitation. However, high levels of silica werestill maintained for greater than 2 hours.

Run 2 and run 3 illustrate the use of liquor of high alumina content andit will be noted that the high alumina and caustic concentrations haveinhibited sodium alumino-silicate precipitation.

Since modifications within the spirit and scope of the invention may bereadily effected by persons skilled in the art, it is to be understoodthat the invention is not limited to the particular embodimentsdescribed hereinabove or by way of the particular examples hereinabove.

It is also to be understood that there will be many possible physicalarrangements, equipment designs and equipment configurations that may beapplied in the operation of the proposed process. Persons skilled in theart will readily effect the use of equipment technology combinations andflowsheet schemes commonly applied in the chemical engineering andmetallurgical industries, and in the Bayer process, in the applicationof the process described herein, by merely following normal processes oftestwork to define optimum parameters for the specific circumstancesunder consideration and engineering design.

In the claims which follow and in the preceding description of theinvention, the words “comprising” and “comprises are used in the senseof the word “including”, ie the features referred to in connection withthese words may be associated with other features that are not expresslydescribed.

What is claimed is:
 1. A process for the removal of reactive silica froma feedstock to the digestion step of the Bayer process for themanufacture of alumina, comprising the steps of: (a) contacting theBayer process feedstock with a caustic liquor under process conditionswhich result in dissolving and stabilizing at least 50% of the silicainto solution at a level of at least 3 gpL and without significantprecipitation from solution of dissolved silica, said process conditionscomprising: (i) caustic liquor composition: caustic strength (as Na₂CO₃)greater than 250 gpL alumina concentration (as Al₂O₃) greater than 125gpL; (ii) contact temperature with caustic liquor: 60° C.-125° C.; (iii)contact time with caustic liquor: greater than 20 minutes; to produce asilica bearing liquor and a solid residue; (b) separating the silicabearing liquor from solid residue of step (a) under conditions which donot promote significant precipitation of the silica; and (c)precipitating aluminosilicates from the separated silica bearing liquorfrom step (b), and forming a solid aluminosilicate product which is freeof a majority of all other components of the feedstock.
 2. The processdefined in claim 1 wherein the silica dissolution step (a) results indissolution of less than 20% by weight of alumina in the feedstock. 3.The process defined in claim 1 wherein the step of precipitatingaluminosilicates includes at least one of the steps of addition of seedto the silica bearing liquor, heating the silica bearing liquor,diluting the silica bearing liquor, and adding additives to the silicabearing liquor.
 4. The process defined in claim 1 wherein the processconditions of silica dissolution step (a) result in dissolving andstabilizing at least 80% of the silica into solution.
 5. The processdefined in claim 4 wherein the process conditions of silica dissolutionstep (a) result in dissolving and stabilizing at least 90% of the silicainto solution.
 6. The process defined in claim 1 wherein the level ofdissolved and stabilized silica in dissolution step (a) is 6-15 gpL. 7.The process defined in claim 1 wherein the caustic strength of thecaustic liquor (as Na₂CO₃) in silica dissolution step (a) is greaterthan 300 gpL.
 8. The process defined in claim 7 wherein the causticstrength of the caustic liquor (as Na₂CO₃) is greater than 350 gpL. 9.The process defined in claim 1 wherein the alumina concentration of thecaustic liquor (as Na₂CO₃) in silica dissolution step (a) is greaterthan 150 gpL.
 10. The process defined in claim 9 wherein the aluminaconcentration of the caustic liquor (as Al₂O₃) in silica dissolutionstep (a) is greater than 175 gpL.
 11. The process defined in claim 1wherein the temperature of the silica dissolution step is 80° C.-105° C.12. The process defined in claim 1 wherein the contact time of thesilica dissolution step (a) is at least 1 hour.
 13. The process definedin claim 12 wherein the contact time is 2-4 hours.
 14. The processdefined in claim 1 wherein the dissolved silica content of the causticliquor supplied to the silica dissolution step is less than 2 gpL. 15.The process defined in claim 1 wherein the silica bearing liquor insilica dissolution step (a) has a solids loading of 50-700 gpL.
 16. Aprocess as defined in claim 1, wherein the feedstock comprises greaterthan 5% by weight boehmite or diaspore.
 17. The process defined in claim1 additionally comprising subjecting the solid residue obtained in step(b) to the Bayer process for digestion of bauxite contained therein.